Until recently no truly iron-based alloys were used for high strength, long-term applications above 871.degree. C. (1600.degree. F.). However, the relatively new iron-based ODS (oxide-dispersion-strengthened) alloy INCOLOY alloy MA 956 can be used up to 1204.degree. C. (2200.degree. F.). (INCOLOY is a trademark of the Inco family of companies.) Since all previous alloy systems used in this temperature range were cobalt- or nickel-based, it was quite adequate to braze these alloys with compatible cobalt- or nickel-based brazing alloys. However, the use of cobalt- or nickel-based systems to braze an iron-based alloy for use at such elevated temperatures would likely result in Kirkendall porosity during service and loss of physical properties in the brazement.
INCOLOY alloy MA 956 is an oxide-dispersion-strengthened, iron-chromium-aluminum alloy produced by mechanical alloying. The nominal chemical composition is shown in Table I. The alloy is strengthened by a yttrium oxide dispersoid that remains stable at temperatures up to the high melting point of the material. The alloy combines strength at high temperatures with excellent resistance to oxidation, carburization, and hot corrosion. Accordingly, these attributes make it an especially suitable material for gas-turbine combustion chambers, components of advanced energy-conversion systems, and other applications involving rigorous service conditions.
TABLE I ______________________________________ NOMINAL CHEMICAL COMPOSITION OF INCOLOY* ALLOY MA 956 (Wt. %) ______________________________________ Iron 74 Chromium 20 Aluminum 4.5 Titanium 0.5 Yttrium Oxide(Y.sub.2 O.sub.3) 0.5 ______________________________________
Clearly, a brazing alloy compatible with iron-based alloy systems is desirable and necessary.